In the optical communications space, demand for ever-increasing bandwidth has lead to the development of systems operating at a nominal line rates of 10 Gb/s and 40 Gb/s. One such system is the Optical Transport Network (OTN), which is described in general terms in ITU-T G.872. ITU-T G.709 provides the network interface definitions for the OTN.
In general, G.709 defines a set of Optical Transport Units (OTUs) which provide overhead, payload capacity and Forward Error Correction (FEC) for respective different bandwidths. As may be seen in FIG. 1, three OTUs (OTU-1, OTU-2 and OTU-3) are defined, each of which is composed of a 4080 column frame 2 consisting of 16 columns of Operations And Maintenance (OAM) overhead 4; 3808 columns of payload 6; and 256 columns of FEC 8. Taken together, the OAM overhead 4 and payload 6 form a 3824-column Optical Data Unit (ODU) 10 which can be MUXed into a higher orderOTU frame 2. The line rate of each OTU is selected to facilitate bit-transparent transport of conventional SONET/SDH signals, as well as multiplexing of four lower-order ODUs. Thus, an OTU-1 operates at 2.66 Gb/s to support a 2.488 Gb/s OC-48 signal. An OTU-2 frame operates at 10.709 Gb/s to support a 9.953 Gb/s OC-192 signal, or mux four ODU-1s. An OTU-3 frame operates at 43.018 Gb/s to support a 39.813 Gb/s OC-768 signal, or mux four ODU-2s. 
A limitation of the above arrangement is that some constant bit rate (CBR) clients demand bit-transparent transport of signals that do not conform to SONET/SDH line rate conventions. For example, 10 G Ethernet PHY-layer signals operate at a line rate of 10.3 Gb/s. Various methods have been proposed for compressing a 10 G Ethernet-PHY signal to facilitate transport at the OC-192 line rate. See, for example, Applicant's U.S. Pat. No. 6,944,163, which issued Sep. 13, 2005. However, in each of these methods, the process of compressing and de-compressing Ethernet media access control (MAC) frames introduces artefacts (e.g. null-frames removed, inter-frame gaps compressed etc.) into the recovered Ethernet signal. In some cases, these artefacts produce an undesirable degradation in the signal quality, which has led affected clients to demand true bit-transparent transport of their signal traffic. In this respect, “bit transparent” implies that an input signal can be mapped into a suitable container, transported across the network, and then de-mapped to recover the original signal, without introducing artefacts that are detectable by the client.
One known method of accommodating this requirement is by overclocking an OTU-2 signal to provide an OTU2+ frame, in which the line rate is increased to accommodate a higher-rate client signal. For example, increasing the line rate of the OTU-2 signal from 10.709 Gb/s to 11.095 Gb/s enables the resulting OTU2+ to support transparent mapping of 10 G Ethernet PHY-layer client signals. The overclocked OTU-2 (OTU-2+) signal is a logical OTU-2 signal, which can therefore be processed through a conventional OTU-2 capable Application Specific Integrated Circuit (ASIC). However, this solution suffers a disadvantage that a different line rate is required to support transparent mapping of different client signals, which leads of a plurality of different OTN frame rates.
An alternative solution is to use a non-OTN frame to provide transparent mapping of client traffic. This is inevitably a proprietary solution, which defeats the interoperability and network management benefits of adhering to a standards-based transport-layer solution.
While the above limitations are discussed in terms of the OTN described in ITU-T G.872 and G.709, it will be appreciated that the same limitations will be encountered in any optical communications system in which client signal traffic is muxed into a hierarchy of fixed-size containers for transport through the network.
The above discussion illustrates a specific example of a more general problem, in that the lines rates demanded for client traffic may or may not correspond with the line rates used within the transport network. Traditionally, this problem has been addressed by designing transport network standards in which line rates are selected based on assumptions about the client traffic. The OTN described in ITU-T G.872 and G.709 is a typical example of this, in that the OTN line rates are selected on the assumption that the client traffic will run at SONET line rates. Transparent transport of higher speed client traffic forces the transport network operate at correspondingly higher line rates, and this is typically accomplished by either overclocking an existing transport network, or providing an alternate (usually proprietary) transport network solution.
However, it is entirely possible, even likely, that the most cost effective line rates (e.g. in terms of cost/bit/km) within the transport network are entirely different from those suggested by the demands of client traffic. For example, optical communications networks currently being deployed are designed to run at nominal lines rates of 10 Gb/s and 40 Gb/s, per channel, primarily because these line rates directly accommodate SONET and Ethernet client traffic. However, with current optical networking technology, line rates of between 20 and 30 Gb/s, per channel, may be significantly more cost-effective for the transport network provider.
Accordingly, a flexible multi-rate MUX enabling bit-transparent transport of client traffic having a wide range of line rates through an optical network would be highly desirable.
Techniques enabling transport network line rates to be provisioned independently of client traffic line rates would also be highly desirable.